This invention relates to a recording system for use with an acoustic velocity logging tool.
In the exploration for petroleum and other minerals, well logging techniques are employed to determine the character of subsurface formations penetrated by a borehole. One technique of logging these characteristics employs a logging tool including a transmitter of acoustic pulses and a receiver, or receivers, for detecting these acoustic pulses after they have traversed the subsurface formations adjacent the tool. By measuring the traveltime of acoustic energy from the transmitter through the formation to the receiver, an indication of the velocity of acoustic energy through the subsurface formation is obtained and this acoustic velocity is an indication of the character of the subsurface formation. By recording the acoustic velocity in correlation with the depth of borehole as the tool moves through the borehole, there is obtained a log of the characteristics of the subsurface formations as a function of depth. Such logs are quite useful, particularly in the exploration for oil-bearing subsurface strata.
In U.S. Pat. No. 3,302,166 to Joseph Zemanek, Jr., there is disclosed an acoustic velocity logging system wherein a downhole tool comprises a plurality of transducers including a transmitter and multiple receivers. The receivers are spaced at different distances from the transmitter such that an acoustic pulse from the transmitter arrives at the receivers by way of different travel paths through the formation surrounding the borehole. Upon generation of an acoustic pulse by the transmitter, a transmitter trigger pulse is transmitted uphole by way of a conductor in the logging cable. The signals produced by the receivers in response to the arrival of the acoustic pulse at the receivers are also transmitted uphole by way of conductors within the logging cable. The logging system is provided with downhole gating circuits which permit the first acoustic pulse to be detected and sent uphole by a first receiver, the second acoustic pulse to be detected and sent uphole by a second receiver, the third acoustic pulse to be detected and sent uphole by a third receiver, and the fourth acoustic pulse to be detected and sent uphole by a fourth receiver. The cycle is then repeated with successive acoustic pulses being detected and transmitted uphole by successive ones of the receivers. The time interval between the generation of each acoustic pulse and the detection of that pulse at a particular receiver is recorded. These time intervals are combined to provide an indication of acoustic velocity of formations adjacent the logging tool. In such a system, the influence of variations on the acoustic velocity measurement caused by the borehole medium as the acoustic pulses travel to and from the borehole tool is eliminated so that the measurements are dependent solely upon the character of the subsurface formations surrounding the borehole.